1. Field of the Invention
The present invention relates to a method and device for making measurements characterizing geological formations in a horizontal borehole or a borehole slightly slanting with respect to the horizontal, formed from an underground way, such as a gallery, a tunnel etc.
The device of the invention finds its application particularly in the study of formations where galleries or tunnels are being pierced, so as to facilitate the advance of the cutting systems used.
2. Description of the Prior Art
A device is known which is used in coal mines for exploring the formations in front of a gallery in the course of construction for defining the contours of a seam and to locate prior to mining the gas pockets which are likely to explode. This device includes a machine for horizontal boring formed essentially by a drilling tool fixed to the end of a hollow rod, a system for driving the rod, and a measuring device formed of a probe with a cross section less than that of the drilling rod, fixed to the end of a cable transmitting the data measured by the probe.
With a hole drilled beforehand, the probe is introduced into the drilling rod and pushed to the end of the rod by pumping means. This prior device is described in the U.S. Pat. No. 4,498,532.
This type of device lends itself well to investigations carried out using probes which may operate inside a tube or a rod, generally made from metal, and so to the use of radiation probes: gamma, neutron rays whose penetration power is very limited. The distance over which a probe can be driven into the formations is limited to the relatively small length of the tube unless it is formed of a string of rods which are progressively connected to each other. But in this case, the previous step for conveying the probe to the bottom of a borehole of several hundreds of meters in length is very long and that considerably delays the effective taking of measurements. Furthermore, the necessity during the withdrawal phase to remove the successive sections of the rod string, involves stoppages and it is impossible to carry out continuous measurements.
Another known method consists in fixing a probe to the end of a rod having a lateral outlet and in connecting the electric connectors of the probe to a cable which extends to the outside through this side outlet. The rod provided with its probe is engaged in the borehole and as it is gradually lowered tube sections are added and the cable is unwound. To the drawbacks already mentioned relating to the slowness of the operations for connecting the tube sections tegether, is added the presence of the cable on the outside, which greatly complicates the construction of a sleeve for sealingly closing the borehole in that this latter may not be filled with the liquid required for coupling an acoustic measuring probe with the formations, for example.